I found a quote from George Washington's farewell address that was short and thought-provoking. It fits well with the historic comments as the game opens.
“The alternate domination of one faction over another, sharpened by the spirit of revenge, natural to party dissension, which in different ages and countries has perpetrated the most horrid enormities, is itself a frightful despotism. But this leads at length to a more formal and permanent despotism. The disorders and miseries which result gradually incline the minds of men to seek security and repose in the absolute power of an individual; and sooner or later the chief of some prevailing faction, more able or more fortunate than his competitors, turns this disposition to the purposes of his own elevation, on the ruins of public liberty.”
--- George Washington, President of the United States farewell address, 1796.
I've added it as TXT_KEY_QUOTE_in CIV4GameText_Credits_Readme_Hints, and also extended CIV4Hints in Assets/XML/GameInfo to add quote 51.